For a long time, deep-sea investigation relied on autonomous bottom landers. Landers call vary in size from 200 kg weight to more than 2 t for the heaviest scientific landers and are used during exploration cruises oil medium periods, from one week to some months. Today, new requirements appear. Scientists want to understand in detail the phenomena outlined during exploration cruises, to elaborate a model for future forecasting. For this, it is necessary to deploy instrumentation at a precise location often for a long period. A new mode of ocean science investigation Using long-term seafloor observatories to obtain four dimensional data sets has appeared. Although this concept has been proposed for many years, the high level of investment required limits the number of projects implemented. Only multi-disciplinary programs, supported by a strong social requirement were funded. Some observatories have been deployed.